The integer and fractional quantum Hall effects (IQHE and FQHE) are canonical examples of 2D topological phases. Being incompressible liquid in the bulk, available low-energy propagating excitations are only at the edge: gapless chiral edge channels. The excitations therein are electrons in the IQHEs and fractional quasiparticles in the FQHEs. Various collective phenomenon can emerge when interactions take place between coexisting multiple edge channels. For example, upstream (anti-chiral) neutral edge modes in hole-conjugate FQHEs (p+½<ν<p+1, where p=0,1,2… and filling factor ν) exemplified the roles of inter-channel random tunneling and Coulomb interaction. Here, employing low-frequency shot noise measurement as our experimental probe, we first looked solely at Coulomb interaction between two channels at ν=2 IQHE (no inter-channel tunneling) and demonstrated charge fractionalization of chiral electrons in emerging downstream neutral excitations (with zero net current).

Furthermore, we revealed the presence of upstream neutral edge modes not only in hole-like FQHEs, as was theoretically-predicted, but in all tested particle-like FQHEs (p<ν<p+½) – but not in integer fillings. Moreover, and quite surprisingly, the incompressible bulk of the FQHEs was found to transport energy, via bulk neutral modes; though weaker than along the edge. The presence of various neutral modes may imply their unfavorable roles as potential decoherers in the vanishing quantum coherence of fractional quasiparticles, which has been preventing us studying their expected fractional exchange statistics. Hence, understanding the properties of the neutral modes, either edge or bulk, may allow us to control decoherence and thus to conclusively observe quantum oscillations of the fractional quasiparticles.